1. Field of the Invention
The invention relates to liquid crystal display devices and somewhat more particularly to liquid crystal display devices having a layer of a liquid crystal material enclosed between two carrier plates wherein said liquid crystal material includes a nematic carrier substance and at least one chiral additive which induces a cholesteric phase characterized by a twisting in such layer and a method of operating such display device.
2. Prior Art
Liquid crystal display devices having threshold voltages dependent on a finite temperature operating range and having a layer of a liquid crystal material positioned between the carrier plates of such device wherein the liquid crystal material includes a nematic carrier and at least one chiral additive or dopant which induces a cholesteric phase characterized by twisting in the liquid crystal layer are known in a multitude of embodiments; cf. in this regard, for example, DT-OS No. 2,727,562.
When one dopes a nematic liquid crystal material with a chiral compound, the nematic material experiences a characteristic twisting. In such an admixture, the pitch and the direction of rotation of this helix (twisting) depend not only on the effective twisting or rotary power of the dopant and the concentration thereof, but also depend on the temperature experienced by the admixture. Since such helix parameters (i.e., pitch and direction of rotation) in turn influence important characteristic data of the induced cholesteric phase, such as for example, the threshold field strengths required for phase transitions, the necessary switching voltages, for example, also change with the operating temperature.
Such threshold voltage behavior does not have a disruptive effect on the display device, if one drives the device, as a precaution, with a sufficiently high voltage. Such a precaution however increases the power dissipation and and is useful only to a very limited degree, especially in multiplexable display devices because the danger of so-called cross-talk increases with increased switching voltages.
Heretofore suggestions have been made to reset or relate switching voltages to operating temperatures. Control loops suitable for such purposes not only generally demand a significant circuit outlay but can only actually be relatively simply realized when the threshold field strengths have a relatively constant temperature drift. However, such a temperature function has not yet been attained in practically significant liquid crystals with induced cholesteric phases. (Although a threshold voltage which is linearly dependent on temperature has been reported by J. J. Wysocki et al in Liquid Crystals and Ordered Fluids, 1970, pages 419-444, Plenum Press, New York, London, wherein an investigation was based on a binary mixture consisting exclusively of cholesteric components).